In tungsten chemical processing, impure sodium tungstate solutions are generally obtained by caustic digestion of tungsten ores. The impure sodium tungstate solutions are subjected to arsenic, phosphorus and silica removal. This removal process can involve the addition of magnesium chloride and ammonium hydroxide to the sodium tungsten solutions to precipitate the named impurities. The resulting purified sodium tungstate solutions contain relatively high ammonia concentrations. During subsequent processing, the pH of the sodium tungstate solutions is adjusted to about 3.5 from about 9.4 before removing molybdenum. During this pH adjustment, insoluble sodium ammonium paratungstate double salts can form in solutions of relatively high ammonia concentrations. Also, the ammonia in the sodium tungstate ends up in the waste streams and causes pollution problems.
Therefore, a process to remove ammonia from the sodium tungstate solutions would be highly desirable.